The present invention relates to two stroke internal combustion engines.
In conventional two stroke internal combustion engines, the or each cylinder is provided with a series of transfer ports, typically from three to six, through which air or air/fuel mixture may be pumped to sweep the combustion products from the combustion chamber. In engines produced hitherto, the transfer ports are arranged such as to maximise the proportion of combustion products swept from the combustion chamber thus producing maximum power. However; particularly at part load, the flow of scavenging air may be short circuited, so that not all the combustion products will be swept from the combustion chamber, with consequent reduction in fuel efficiency and engine smoothness. Other adverse effects would include increased emission of noxious substances or excessive cooling of exhaust gases.
The present invention provides valve means by which various combinations of the transfer ports may be closed or partially closed to modify the flow of scavenging air under particular operating conditions, thereby reducing short circuiting of the air flow and improving sweeping of the combustion products from the combustion chamber.